Thermally stable high nitrile resins and method for producing the same

ABSTRACT

The present invention relates to a method for stabilizing thermoplastic nitrile resins against thermal discoloration by the inclusion in the resins of acid esters of certain polycarboxylic acid compounds in amounts of from 0.1 to 1.5 parts by weight per 100 parts of resin.

The present invention relates to a method for stabilizing thermoplastic nitrile resins against discoloration resulting from thermal degradation during processing. More particularly, this invention relates to the stabilization of thermoplastic copolymers and graft copolymers containing a high proportionate amount of olefinic nitrile against thermal discoloration by the inclusion of minor amounts of monoesters of certain organic dicarboxylic acids in the polymer.

It has been reported in Faserforschung und Textiltechnik, Vol. 21, No. 3, pages 105-110 (1970), that maleic acid and some of its derivatives are useful as inhibitors for the thermal discoloration of polyacrylonitrile. They mention "the complete indifference of the maleic acid esters" for this purpose. We have discovered that the half esters of maleic acid or fumaric acid are unique for the stabilization of thermoplastic olefinic nitrile resins against thermal discoloration in spite of the fact that the diesters have been found by others to be ineffective for this purpose.

Half esters of maleic acid and fumaric acid are compatible with the nitrile resins of this invention, and they do not impart any appreciable haze or reduced light transmission to the resin, nor do they have any adverse effects on the physical properties of the resins when they are present within specified concentration ranges. Additionally, the low cost of the half esters of maleic acid and fumaric acid and their ready availability make these compounds attractive inhibitors for thermoplastic nitrile resins.

Half esters of maleic acid and fumaric acid can be advantageously incorporated into the resin by intimately mixing or blending with the powdered resin. Improvement in color is observed by adding half esters of maleic acid and fumaric acid in concentrations as low as about 0.1 part per hundred parts of resin, on a weight basis, and although no greater color improvement is obtained with concentrations beyond about 1.5 parts per hundred parts of resin (phr), there appears to be no adverse effect on the properties of the resin with higher concentrations. The greatest degree of color improvement, however, is obtained when the stabilizers of this invention are employed in concentrations ranging from about 0.3 to about 1.2 parts by weight per hundred parts by weight of resin.

The half esters of maleic acid and fumaric acid which are useful in the present invention include those conforming to the structure HOOCCH = CHCOOR₃ wherein R₃ is a hydrocarbon group having from 1 to 20 carbon atoms.

The resins useful in this invention are composed of polymers of an olefinically unsaturated nitrile, another monomer component copolymerizable with said olefinically unsaturated nitrile, and optionally a diene rubber.

The polymers are those produced by polymerizing a major proportion of a monounsaturated nitrile, such as acrylonitrile, and a minor proportion of another monovinyl monomer component copolymerizable with said nitrile, in an aqueous medium, preferably in the presence of a preformed diene rubber which may be a homopolymer or a copolymer of a conjugated diene monomer.

The conjugated diene monomers useful in the present invention include butadiene-1,3, isoprene, chloroprene, bromoprene, cyanoprene, 2,3-dimethyl-butadiene-1,3, 2-ethyl-butadiene-1,3, 2,3-diethyl-butadiene-1,3, and the like, and others. Most preferred for the purpose of this invention are butadiene and isoprene because of their ready availability and their excellent copolymerization properties.

The olefinically unsaturated nitriles useful in the present invention are the alpha,beta-olefinically unsaturated mononitriles having the structure ##STR1## wherein R is hydrogen, a lower alkyl group having from 1 to 4 carbon atoms, or a halogen. Such compounds include acrylonitrile, alpha-chloroacrylonitrile, alpha-fluoroacrylonitrile, methacrylonitrile, ethacrylonitrile, and the like. The most preferred olefinically unsaturated nitriles in the present invention are acrylonitrile and methacrylonitrile, and mixtures thereof.

The other monovinyl monomer component copolymerizable with the olefinically unsaturated nitriles which are useful in this invention includes one or more of the vinyl aromatic monomers, esters of olefinically unsaturated carboxylic acids, vinyl esters, vinyl ethers, alpha-olefins, and others.

The vinyl aromatic monomers include styrene, alpha-methyl styrene, the vinyl toluenes, the vinyl xylenes, and the like. Most preferred is styrene.

The esters of olefinically unsaturated carboxylic acids include those having the structure ##STR2## wherein R₁ is hydrogen, an alkyl group having from 1 to 4 carbon atoms, or a halogen, and R₂ is an alkyl group having from 1 to 6 carbon atoms. Compounds of this type include methyl acrylate, ethyl acrylate, the propyl acrylates, the butyl acrylates, the amyl acrylates, and the hexyl acrylates; methyl methacrylate, ethyl methacrylate, the propyl methacrylates, the butyl methacrylates, the amyl methacrylates, and the hexyl methacrylates; methyl alpha-chloroacrylate, ethyl alpha-chloroacrylate, and the like. Most preferred in the present invention are methyl acrylate, ethyl acrylate, methyl methacrylate, and ethyl methacrylate.

The alpha-olefins useful in the present invention are those having at least 4 and as many as 10 carbon atoms and having the structure ##STR3## wherein R' and R" are alkyl groups having from 1 to 7 carbon atoms, and more specifically preferred are alpha-olefins such as isobutylene, 2-methyl butene-1, 2-methyl pentene-1, 2-methyl hexene-1, 2-methyl heptene-1, 2-methyl octene-1, 2-ethyl butene-1, 2-propyl pentene-1, and the like. Most preferred is isobutylene.

The vinyl ethers include methyl vinyl ether, ethyl vinyl ether, the propyl vinyl ethers, the butyl vinyl ethers, methyl isopropenyl ether, ethyl isopropenyl ether, and the like. Most preferred are methyl vinyl ether, ethyl vinyl ether, the propyl vinyl ethers, and the butyl vinyl ethers.

The vinyl esters include vinyl acetate, vinyl propionate, the vinyl butyrates, and the like. Most preferred is vinyl acetate.

Polymers of particular utility in this invention and details of their methods of preparation are described in U.S. Pat. Nos. 3,426,102, 3,586,737 and 3,763,278.

The polymers useful in this invention are those prepared by the polymerization of 100 parts by weight of (A) at least 50% by weight of at least one nitrile having the structure ##STR4## wherein R has the foregoing designation and (B) up to 50% by weight based on the combined weight of (A) and (B) of at least one member selected from the group consisting of (1) styrene, (2) an ester having the structure ##STR5## wherein R₁ and R₂ have the foregoing designations, (3) an alpha-olefin having the structure ##STR6## wherein R' and R" have the foregoing designations, (4) a vinyl ether selected from the group consisting of methyl vinyl ether, ethyl vinyl ether, the propyl vinyl ethers, and the butyl vinyl ethers, and (5) vinyl acetate, in the presence of from 0 to 40 parts by weight of (C) a rubbery polymer of a conjugated diene monomer selected from the group consisting of butadiene and isoprene and optionally at least one comonomer selected from the group consisting of styrene, a nitrile monomer having the structure ##STR7## wherein R has the foregoing designation, and an ester having the structure ##STR8## wherein R₁ and R₂ have the foregoing designations, said rubbery polymer containing from 50 to 100% by weight of polymerized conjugated diene and from 0 to 50% by weight of comonomer.

Preferably, component (A) should be present in from about 60 to 90% by weight based on the combined weight of (A) and (B) and the rubber polymer (C) should contain more than 50% by weight of conjugated diene and more preferably from 60 to 90% by weight of the conjugated diene.

The polymers useful in this invention can contain in addition to the half ester of maleic acid other compounding ingredients and additives, pigments, colorants, stabilizers, etc., as is well known in the art so long as the balance between impact strength, flexural strength, tensile strength, processability, heat-distortion temperature, and the like, is not affected to such a degree that the article is no longer useful for its intended purpose.

Polymeric products of the present invention are thermoplastic materials which can be thermoformed into a wide variety of useful articles in any of the conventional ways employed with known thermoplastic polymeric materials, such as by extrusion, milling, molding, drawing, blowing, and the like. These polymers have excellent solvent resistance and their impact strength and low permeability to gases and vapors make them very useful in the packaging industry, and they are particularly useful in the manufacture of bottles, film, and other types of containers for liquids and solids.

The following examples will further illustrate the present invention in greater detail, however, it is to be understood that the scope of this invention is not to be limited by these examples.

The following polymers were prepared and used as designated in the specific examples.

    ______________________________________                                         Polymer A                                                                      (1)  Preparation of Elastomer                                                  Components           Parts by Weight                                           ______________________________________                                         water                200                                                       butadiene            75                                                        styrene              25                                                        t-dodecyl mercaptan  0.7                                                       ethylenediamine tetraacetic                                                                         0.1                                                       acid, tripotassium salt                                                        (Hampol K.sub.3 -120)                                                          Gafac RS-710*        0.7                                                       potassium persulfate 0.2                                                       polydimethyl siloxane                                                                               0.01                                                      (50% solution)                                                                 (antifoam (Dow FG-10))                                                         ______________________________________                                          *α-tridecyl-ωhydroxypoly(oxyethylene) mixture of dihydrogen        phosphate and mnohydrogen phosphate esters having an acid number of 58 to      70, manufactured by GAF Corporation.                                     

To 200 parts of water were added 0.7 part of Gafac RS-710 and 0.1 part of Hampol K₃ -120. The ingredients were stirred until well dispersed, and the pH was adjusted to 6.7 with a 25% aqueous solution of KOH. 0.2 Part of potassium persulfate was added to the reactor along with 0.7 part of t-dodecyl mercaptan dissolved in 25 parts of styrene. The reactor was purged with nitrogen, evacuated and 75 parts of butadiene were added to the mixture. The reactor was slowly heated to 60° C, and the reaction was allowed to continue for 19 hours at which time the solids level of the latex reached 27.3% by weight. 0.01 Part of Dow FG-10 antifoam agent was added to the reactor with the aid of nitrogen pressure. The reactor was slowly vented and vacuum stripping was begun while the temperature remained at 60° C. The unreacted monomers were removed over a period of 2 hours under a vacuum of 20" Hg. The reactor was brought to atmospheric pressure with nitrogen, and the latex was allowed to cool. The latex was then filtered through cheesecloth, and the final total solids of the filtered latex measured 29% by weight.

    ______________________________________                                         (2)  Preparation of Graft Polymer                                              Components           Parts by Weight                                           ______________________________________                                         water                250 (includes water                                                            in elastomer)                                             acrylonitrile        75.5                                                      styrene              24.5                                                      pentaerythritol tetrakis                                                                            2.1                                                       mercaptopropionate                                                             (Evans Q-43)                                                                   elastomer from step (1)                                                                             51.7 (5 parts rubber)                                     (29% T.S.)                                                                     Gafac RS-710         1.2                                                       tridecyloxypoly      0.8                                                       (ethyleneoxy) ethanol                                                          (Emulphogene BC-420)                                                           citric acid          0.2                                                       2,2' azobis-(α,γ-dimethyl                                                               0.165                                                     valeronitrile) (Vazo 52)                                                       (initiator)                                                                    bis-(mixed mono- and 0.01                                                      di-nonyl phenyl phosphite)                                                     (Naugard PHR)                                                                  butylated hydroxytoluene                                                                            0.01                                                      (Naugard BHT)                                                                  ______________________________________                                    

155 Parts of water in addition to the 37 parts of water present in the elastomer latex were charged to a reactor. To this was added with slow mixing a water-emulsifier solution containing 1.2 parts of Gafac RS-710, 0.8 part of Emulphogene BC-420, and 0.2 part of citric acid in 30 parts of water.

This was followed by the addition of 51.7 parts of the above elastomer and 8.25 parts of water with stirring. A mixture of 8 parts of acrylonitrile and 2 parts of styrene was added continuously over a period of 25 to 30 minutes. Agitation was stopped, the contents of the vessel were allowed to stand for 2 hours, and the latex was then filtered through cheesecloth.

The rubber-monomer mixture prepared above was then added to a second reactor and the first container was then rinsed into the reactor with 20 parts of water. The reactor was purged with nitrogen while being heated to 65° C. 0.085 Part of Vazo 52 dissolved in 0.5 part of acrylonitrile was then added to the reactor. The monomer addition of 67.5 parts of acrylonitrile, 22.5 parts of styrene, 2.1 parts of Evans Q-43, 0.01 part of Naugard PHR, and 0.01 part of Naugard BHT was started immediately under nitrogen purge and was continued over a period of 4 hours and 20 minutes. 0.045 Part of Vazo 52 in 0.5 part of acrylonitrile was added after 1 and 1/2 hours and 0.035 part of Vazo 52 in 0.5 part of acrylonitrile was added after 3 and 1/2 hours. With each addition of the Vazo 52 solution, 0.5 part of acrylonitrile was used as a rinse. Vacuum stripping was begun after 4 hours and 50 minutes of reaction time. The temperature was held at 65° C while the unreacted monomers were removed under a vacuum of 18" Hg. The total solids content of the resulting latex was 28% by weight. The latex was coagulated in 1 and 1/2 volumes of an aqueous solution of Al₂ (SO₄)₃ .18H₂ O containing 3 grams of the aluminum salt per hundred grams of resin solids, which had been heated to a temperature of 88° to 90° C.

    ______________________________________                                         Polymer B                                                                      Components           Parts by Weight                                           ______________________________________                                         water                225                                                       acrylonitrile        77                                                        styrene              23                                                        Gafac RE-610**       1.35                                                      pentaerythritol tetrakis                                                                            1.65                                                      mercaptopropionate                                                             (Carlisle Q-43)                                                                2,2' azobis-         0.16                                                      (isobutyronitrile)                                                             (Vazo 64)                                                                      butylated hydroxytoluene                                                                            0.01                                                      ______________________________________                                          **A mixture of R--O--(CH.sub.2 CH.sub.2 O--).sub.n PO.sub.3 M.sub.2 and        [R--O--(CH.sub.2 CH.sub.2 O--).sub.n ].sub.2 PO.sub.2 M wherein n is a         number from 1 to 40, R is an alkyl or alkaryl group, and M is hydrogen,        ammonia or an alkali metal, produced by GAF Corporation.                 

195 Parts of water were charged to a reactor followed by the addition of 1.35 parts of Gafac RE-610. An additional 20 parts of water were utilized to rinse the Gafac RE-610 into the reactor.

The reactor, containing the emulsifier solution, was purged with nitrogen and heated to 70° C. The precharge, consisting of 1 part of styrene in 8 parts of acrylonitrile, was charged to the reactor followed by the addition of an additional 10 parts of water. After a period of 15 minutes, 0.1 part of Vazo 64 in 0.5 part of acrylonitrile was added to the reactor. An additional 0.5 part of acrylonitrile was added to rinse the entrance port of the reactor. The remaining water (10 parts) was then added. Another 0.03 part of Vazo 64 was added after a period of 1 and 1/2 hours and 0.03 part after 3 and 1/2 hours. A total of six portions of 0.5 part of acrylonitrile, 0.5 part as a solvent and 0.5 part as a rinse was used each time the initiator was added.

The addition of the monomer-chain transfer solution prepared by adding 1.65 parts of Carlisle Q-43 to 66 parts of acrylonitrile and 22 parts of styrene was begun immediately, and pumping was conducted linearly over a period of 4 hours. The reaction was continued for an additional 45 minutes and was terminated by the addition of 0.01 part of Naugard BHT. The temperature was held at 70° C while the unreacted monomers were removed under a vacuum of 20" Hg. The total solids content of the resulting latex was 29.5%. The latex was coagulated in 1 and 1/2 volumes of water, heated to 88° to 90° C and contained 3 grams of Al₂ (SO₄)₃ .18H₂ O per hundred grams of resin solids.

    ______________________________________                                         Polymer C                                                                      Components           Parts by Weight                                           ______________________________________                                         water                156                                                       acrylonitrile        75                                                        methyl acrylate      25                                                        Gafac RE-610         3                                                         (emulsifier)                                                                   pentaerythritol tetrakis                                                                            1.27                                                      mercaptopropionate                                                             potassium persulfate 0.06                                                      tetraethylenediamine 0.05                                                      tetraacetic acid,                                                              tetrapotassium salt                                                            (41% aqueous solution)                                                         (Hampene K.sub.4 100)                                                          ______________________________________                                    

148 Parts of water were charged to a container. 0.3 Part of Garfac RE-610 was added with stirring. The pH of the mixture was adjusted to 5.1 with NH₄ OH, and the solution was charged to the reactor along with 0.13 part of Carlisle Q-43 dissolved in 7.5 parts of acrylonitrile and 2.5 parts of methyl acrylate. To this was added 0.05 part of Hampene K₄ 100 to complete the initial reactor charge.

The reactor and its contents were purged with nitrogen and sealed. The reactor was then heated to 68° C with stirring. When the temperature had leveled off at 68° C, a solution of 0.006 part of K₂ S₂ O₈ in 1.48 parts of water was added under a nitrogen stream, then the addition of the two solutions, one consisting of 2.7 parts of Gafac RE-610 and 1.14 parts of Carlisle Q-43 dissolved in a mixture of 67.5 parts of acrylonitrile and 22.5 parts of methyl acrylate and the other consisting of 0.054 part of K₂ S₂ O₈ in 7.4 parts of water, was begun. Both solutions were pumped in linearly over a 6-hour period. The total reaction time was 8 and 1/4 hours. The final solids content in the latex was 36.5%. The unreacted monomers were removed by vacuum stripping under a pressure of 20" Hg for 2 hours at 68° C. The latex was coagulated in 1 and 1/2 volumes of water containing 3 grams of Al₂ (SO₄)₃ .18H₂ O per hundred grams of resin solids. The coagulation was conducted at 71° C.

POLYMER D

1. Preparation of Elastomer

An elastomer was prepared by reacting the following ingredients for 9 hours at 53° C in a nitrogen atmosphere:

    ______________________________________                                         Components           Parts by Weight                                           ______________________________________                                         butadiene            70                                                        acrylonitrile        30                                                        t-dodecyl mercaptan  0.71                                                      P & G soap flakes    1.4                                                       azobisisobutyronitrile                                                                              0.4                                                       ethylenediamine      0.05                                                      tetraacetic acid                                                               sodium salts of polymerized                                                                         0.1                                                       alkylnaphthalene sulfonic                                                      acids                                                                          water                200                                                       ______________________________________                                    

The final pH of the reaction mixture was 9.5 and the total solids after completion of the reaction was 30%.

2. Preparation of Graft Polymer

A graft resin was prepared utilizing the above elastomer by reacting the following components at 57° C over a period of 3.1 hours:

    ______________________________________                                         Components           Parts by Weight                                           ______________________________________                                         acrylonitrile        75                                                        methyl acrylate      25                                                        elastomer solids     9                                                         (obtained from (1)                                                             above)                                                                         sodium salt of dioctyl                                                                              1.21                                                      sulfosuccinate                                                                 polyvinylpyrrolidone 0.3                                                       pentaerythritol tetrakis                                                                            1.55                                                      (mercaptopropionate)                                                           potassium persulfate 0.06                                                      antifoam (Dow FG-10) <.04                                                      water                230                                                       ______________________________________                                    

All of the above materials were added initially to the reactor with agitation with the exception of the chain transfer agent, which was added 20 minutes after the start of the reaction. Completion of the reaction was determined on the basis of total solids of 28% and a conversion of 85%. The latex was coagulated in methanol with aqueous aluminum sulfate, filtered and dried.

EXAMPLES 1-12

The various half-ester stabilizers were added to the dried powders of the polymers described in A through D above in the amount of 0.5 part by weight per hundred parts of resin, as indicated in Table 1. These resin compositions were then evaluated for their color stability and optical properties by a method based on the use of a Brabender Plasticorder. The method was designed to enable the prediction of the optical properties such as haze, color, and light transmission resulting from processing various resin batches into fabricated articles. It was found that 5- to 7-minute residence time in the Brabender at 200° C is equivalent to the total heat history that the resin would encounter during normal processing.

The instrument used consisted of a dynamometer, Type PL-U33AA (No. 2162-64), and its accompanying roller style measuring head (No. A-30/S.B.). The roller style measuring head was electrically heated and was equipped with a Rosemont temperature controller. Fifty-gram samples of each resin were screened in the Brabender set for a temperature of 200° C and 35 rpm. Four samples, approximately 1.5 grams in weight, were removed at 3-minute intervals from the Brabender for optical measurements. The initial sample was taken 4 minutes after the start of sample loading. Torque values and temperature were also noted at the 4-minute mark. Discs having a uniform thickness of 40 mils were prepared for optional measurements by pressing the fused polymer under mild conditions. The conditions selected for the pressing of discs from Brabender samples did not contribute significantly to measured optical values. The disc was prepared by placing a mold 40-mils thick and 11/4 inches in diameter in a press between two parallel mirror-finished, chrome-plated brass platens, 1/8-inch thick. The platens were heated to 154° C. A 1.5-gram sample of the cooled resin removed at the various times from the Brabender was placed in each of the four cavities of the mold or in two cavities diagonally across from one another if only two discs were being processed. The heated plates containing the resin and the mold therebetween were positioned in the press so that the melt samples were pressed between the two highly polished surfaces. Fifteen thousand pounds' ram pressure was applied and held for 5 to 10 seconds. The pressure was then released and the entire unit transferred to a cold press where 20,000 pounds' ram pressure was applied. After approximately 30 seconds, the plates were sufficiently cool to handle. The unit was then removed and the discs were removed from the cavities. The percent light transmission was measured with the Hunter Lab Colormeter, and the yellowness index and percent haze were determined by ASTM Standard Methods D 1925-70 and D 1003-61, respectively.

It is apparent from the data shown in Table 1 that the half esters of the present invention are unique among the compounds tested, including the magnesium dodecyl maleate described as being a stabilizer for polyacrylonitrile in U.S. Pat. No. 2,719,140.

                                      Table 1                                      __________________________________________________________________________                                Stabilizer                                                                             Yellowness Index                                                       Concentration                                                                          (minutes)                                   Example                                                                              Polymer                                                                              Stabilizer     phr (wt.)                                                                              4   7   10  13                              __________________________________________________________________________     1     B     none           0       15.1                                                                               49.6                                                                               74.4                                                                               88.8                            2     B     monobutyl maleate                                                                             0.5     12.9                                                                               19.9                                                                               26.9                                                                               33.0                            3     B     monooctyl maleate                                                                             0.5     12.4                                                                               18.8                                                                               27.3                                                                               33.7                            4     B     monolauryl maleate                                                                            0.5     12.6                                                                               19.2                                                                               28.1                                                                               36.5                            5     B     monocetyl maleate                                                                             0.5     15.3                                                                               22.4                                                                               31.3                                                                               40.1                            6     B     magnesium dodecyl maleate                                                                     0.5     13.5                                                                               20.6                                                                               32.3                                                                               44.2                            7     A (2) none           0       12.2                                                                               23.2                                                                               33.7                                                                               42.0                            8     A (2) monooctyl maleate                                                                             0.5      8.9                                                                               11.4                                                                               13.6                                                                               15.9                            9     D (2) none           0       10.8                                                                               15.7                                                                               22.2                                                                               27.9                            10    D (2) monooctyl maleate                                                                             0.5     10.2                                                                               13.7                                                                               18.1                                                                               22.1                            11    C     none           0       11.1                                                                               27.9                                                                               49.8                                                                               66.8                            12    C     monooctyl maleate                                                                             0.5     10.3                                                                               15.0                                                                               19.4                                                                               22.4                            __________________________________________________________________________

EXAMPLES 13-19

The procedures described in Examples 1-12 were repeated using varying amounts of monooctyl maleate in polymer B. The results are given in Table 2.

                                      Table 2                                      __________________________________________________________________________                            Stabilizer                                                                             Yellowness Index                                                       Concentration                                                                          (minutes)                                       Example                                                                              Polymer                                                                              Stabilizer phr (wt.)                                                                              4   7   10  13                                  __________________________________________________________________________     13    B     none       0       15.6                                                                               47.4                                                                               75.3                                                                               87.5                                14    B     monooctyl maleate                                                                         0.1     12.8                                                                               25.6                                                                               43.4                                                                               61.5                                15    B     monooctyl maleate                                                                         0.2     11.3                                                                               19.4                                                                               32.0                                                                               43.2                                16    B     monooctyl maleate                                                                         0.4     11.2                                                                               18.5                                                                               27.8                                                                               34.6                                17    B     monooctyl maleate                                                                         0.8     12.4                                                                               18.6                                                                               26.1                                                                               31.2                                18    B     monooctyl maleate                                                                         1.2     13.8                                                                               20.7                                                                               28.2                                                                               33.5                                19    B     monooctyl maleate                                                                         1.6     15.3                                                                               22.9                                                                               30.9                                                                               35.4                                __________________________________________________________________________

EXAMPLES 20-26

The procedures described in Examples 1-12 were repeated using various esters described in the prior art which are outside the scope of the present invention. The results are given in Table 3 wherein a comparison of properties of the resulting products is also made with the control resin and a stabilized resin within the scope of the present invention. It is readily apparent that the stabilized resins according to this invention are superior in properties to those of Examples 20-26 shown in Table 3.

                                      Table 3                                      __________________________________________________________________________                           Stabilizer                                                                             Yellowness Index % Haze                                                Concentration                                                                          (minutes)        (minutes)                       Example                                                                               Polymer                                                                              Stabilizer                                                                              phr (wt.)                                                                              4    7   10  13  4   7   10  13                  __________________________________________________________________________     1      B     none     0       15.1 49.6                                                                               74.4                                                                               88.8                                                                               2.7 3.8 4.0 5.8                 2      B     monobutyl                                                                               0.5     12.9 19.9                                                                               26.9                                                                               33.0                                                                               3.3 3.4 3.5 4.9                              maleate                                                           20     B     dibutyl  0.5     14.8 34.4                                                                               54.2                                                                               69.1                                                                               3.2 2.3 3.6 5.1                              maleate                                                           21     B     di-2-ethyl-                                                                             0.5     16.3 35.1                                                                               55.7                                                                               68.4                                                                               4.1 4.0 4.1 4.4                              hexyl                                                                          maleate                                                           22     B     monooctyl                                                                               0.5     18.0 47.9                                                                               78.8                                                                               90.6                                                                               5.1 4.9 6.8 4.3                              succinate                                                         23     B     monomethyl                                                                              0.5     23.4 58.2                                                                               84.2                                                                               96.0                                                                               3.2 2.9 4.5 5.1                              succinate                                                         24     B     dioctyl  0.5     17.0 45.2                                                                               74.4                                                                               87.3                                                                               4.3 4.2 4.6 4.1                              phthalate                                                         25     B     dimethyl 0.5     15.3 51.4                                                                               81.1                                                                               93.8                                                                               2.3 2.3 2.6 4.7                              fumarate                                                          26     B     dimethyl 0.5     15.9 47.8                                                                               74.7                                                                               88.6                                                                               2.0 2.3 3.2 4.5                              phthalate                                                         __________________________________________________________________________ 

We claim:
 1. A polymer composition stable toward therml discoloration containing from about 0.1 to about 1.5 parts by weight of at least one ester corresponding to the formula HOOCCH = CHCOOR₃ wherein R₃ is a hydrocarbon group having from 1 to 20 carbon atoms and wherein said polymer is composed of 100 parts by weight of the polymerization product of the monomersA. at least 50% by weight of at least one nitrile having the structure ##EQU1## wherein R is hydrogen, a lower alkyl group having from 1 to 4 carbon atoms, or a halogen, and B. up to 50% by weight based on the combined weight of (A) and (B) of at least one member selected from the group consisting of
 1. styrene,2. an ester having the structure ##EQU2## wherein R₁ is hydrogen, an alkyl group having from 1 to 4 carbon atoms, or a halogen, and R₂ is an alkyl group having from 1 to 6 carbon atoms,
 3. an alpha-olefin having the structure ##EQU3## wherein R' and R" are alkyl groups having from 1 to 7 carbon atoms,
 4. a vinyl ether selected from the group consisting of methyl vinyl ether, ethyl vinyl ether, the propyl vinyl ethers, and the butyl vinyl ethers, and5. vinyl acetate, wherein said monomers are copolymerized in the presence of from 0 to 40 parts by weight of C. a rubbery polymer of a conjugated diene monomer selected from the group consisting of butadiene and isoprene and optionally at least one comonomer selected from the group consisting of styrene, a nitrile monomer having the structure ##EQU4## wherein R has the foregoing designation, and an ester having the structure ##EQU5## wherein R₁ and R₂ have the foregoing designations, said rubbery polymer containing from 50 to 100% by weight of polymerized conjugated diene and from 0 to 50% by weight of comonomer.
 2. The composition of claim 1 wherein (A) is acrylonitrile.
 3. The composition of claim 2 wherein (B) is styrene.
 4. The composition of claim 2 wherein (B) is methyl acrylate.
 5. The composition of claim 2 wherein (C) is a copolymer of butadiene and acrylonitrile.
 6. The composition of claim 2 wherein (C) is a copolymer of butadiene and styrene. 